Scalability and QoS in MS-Aloha VANETs: Forced slot re-use versus pre-emption

While IEEE 802.11p has been world-wide recognized as the incumbent MAC solution for VANET, some synchronous slotted solutions still survive not only in the scientific literature but also in standardization community: they represent an opportunity for possible next-generation solutions specifically addressing determinism, as required by safety services. Reversely it is arguable how much a connection-oriented slotted protocol can guarantee the flexibility required by non-safety applications: in fact they are supposed to benefit from statistical multiplexing, which is not natively supported by connection-oriented protocols. This paper presents a preliminary analysis on this issue leveraging on MS-Aloha, a hidden-terminal-free slotted protocol which has been demonstrated to outperform CSMA/CA and has several interesting features: among them, in addition to determinism, a smart slot re-use and a pre-emption mechanism which are here demonstrated to improve the protocol scalability and flexibility.